Insights into ultrafast demagnetization in pseudo-gap half metals

TL;DR

This study investigates ultrafast demagnetization in highly spin-polarized pseudo-gap half metals using femtosecond pump-probe experiments, revealing how electronic structure influences spin-flip processes and demagnetization dynamics.

Contribution

It demonstrates the control of ultrafast demagnetization by tuning the electronic structure in isoelectronic Heusler compounds and related alloys, advancing understanding of spin dynamics in half metals.

Findings

Spin polarization varies between 60% and 86% across studied compounds.

Robustness of the electronic gap affects spin-flip channel availability.

Co-Fe-based alloys can achieve high spin polarization similar to ideal half metals.

Abstract

Interest in femtosecond demagnetization experiments was sparked by Bigot's discovery in 1995. These experiments unveil the elementary mechanisms coupling the electrons' temperature to their spin order. Even though first quantitative models describing ultrafast demagnetization have just been published within the past year, new calculations also suggest alternative mechanisms. Simultaneously, the application of fast demagnetization experiments has been demonstrated to provide key insight into technologically important systems such as high spin polarization metals, and consequently there is broad interest in further understanding the physics of these phenomena. To gain new and relevant insights, we perform ultrafast optical pump-probe experiments to characterize the demagnetization processes of highly spin-polarized magnetic thin films on a femtosecond time scale. Previous studies have suggested shifting the Fermi energy into the center of the gap by tuning the number of electrons and thereby to study its influence on spin-flip processes. Here we show that choosing isoelectronic Heusler compounds (Co2MnSi, Co2MnGe and Co2FeAl) allows us to vary the degree of spin polarization between 60% and 86%. We explain this behavior by considering the robustness of the gap against structural disorder. Moreover, we observe that Co-Fe-based pseudo gap materials, such as partially ordered Co-Fe-Ge alloys and also the well-known Co-Fe-B alloys, can reach similar values of the spin polarization. By using the unique features of these metals we vary the number of possible spin-flip channels, which allows us to pinpoint and control the half metals electronic structure and its influence onto the elementary mechanisms of ultrafast demagnetization.